Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking (FCC) processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
Test methods for fluid catalytic cracking are well established and in some cases commercially available, so long as one is considering the hydrocarbon cracking portion of the process that takes place in the riser. Fixed bed MAT is defined by an ASTM method. The ACE™ fixed fluidized bed and a circulating riser pilot plant test units are sold commercially. These standard methods were designed to give information on the yields of cracked products from hydrocarbon feeds.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons. When the hydrocarbon feedstock contains organic sulfur and nitrogen compounds, the coke also contains sulfur and nitrogen. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes. Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas or steam at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas, such as air. The combustion of these coke deposits can be regarded, in a simplified manner, as the oxidation of C1H1. The products from such combustion are water, carbon monoxide and carbon dioxide.
The waste gas stream from the combustion process is called flue gas. High residual concentrations of carbon monoxide in flue gases from regenerators have been a problem since the inception of catalytic cracking processes. The evolution of FCC has resulted in the use of increasingly high temperatures in FCC regenerators in order to achieve the required low carbon levels in the regenerated catalysts. Typically, present day regenerators now operate at temperatures in the range of about 1100° F. to 1400° F. When no CO oxidation promoter is used, the flue gases may have a CO2/CO ratio in the range of 1-3. The oxidation of carbon monoxide is highly exothermic and can result in so-called carbon monoxide “afterburning” which can take place in the dilute catalyst phase (freeboard region), in the cyclones or in the flue gas lines. Afterburning has caused significant damage to plant equipment. On the other hand, unburned carbon monoxide in atmosphere-vented flue gases represents a loss of fuel value and is ecologically undesirable.
Restrictions on the amount of carbon monoxide which can be exhausted into the atmosphere and the process advantages resulting from more complete oxidation of carbon monoxide have stimulated several approaches to achieve complete combustion, also known as “full burn,” of carbon monoxide in the regenerator. Since the coke often contains nitrogen and sulfur, under full burn conditions, the flue gas also contains NOx and SOx components.
As opposed to complete CO combustion, FCC catalyst regenerators may be operated in an incomplete mode of combustion, and these are commonly called “partial burn” units. Incomplete CO combustion leaves a relatively large amount of coke on the regenerated catalyst which is passed from an FCC regeneration zone to an FCC reaction zone. The relative content of CO in the regenerator flue gas is relatively high, i.e., about 1 to 10 volume percent. A key feature of partial combustion mode FCC is that the heat effect of coke burning per weight of coke is reduced because the exothermic CO combustion reaction is suppressed. This enables higher throughput of oil and lower regenerator temperatures, and preservation of these benefits is essential to the economics of the partial burn FCC process. Under incomplete combustion operation NOx may not be observed in the regenerator flue gas, but sizable amounts of ammonia and HCN are normally present in the flue gas.
Different reactants and products of interest are found in the regenerator compared to gaseous products found in the FCC reactor. When coke is burned, it generates CO2, CO, and H2O at percent levels, and ppm levels of SO2, HCN, NH3 and NO, and lesser amounts of COS, NO2, N2O and other nitrogen oxides. H2 and H2S might also be formed under some circumstances. CO and many of these ppm-level gases are toxic and are regulated emissions. Cracking catalyst additives have been developed to control the concentrations of the emissions in the regenerator flue gases. Development of such important catalyst additives has been hindered however for lack of performance test methods that can both realistically and conveniently emulate the regenerator environment which the catalyst and additives function in, and which also provide the maximum kinetic and chemical information possible about the regeneration system.
While certain fluid bed or circulating pilot plants burn coke on spent catalyst and can be asserted to be relevant to coke combustion and catalysis in the regenerator, the pilot plants were not specifically designed for maximum utility in cracking catalyst regeneration studies. One skilled in the art expects that there will be false positives and false negatives associated with testing done in apparatus that were not specifically engineered to remove artifacts of the processing and systematic errors, especially if there was no deliberate investigation of such possibilities. Indeed, this has been found to be the case. Improvements in analysis of FCC regeneration are therefore needed.
Transient test methods and the results thereof have long been known to provide higher information content than steady state reaction methods in catalysis and reaction engineering. This is because a wide variety of process conditions are inherently employed and because there is the possibility of transient accumulation of species on the catalyst or in the reactor. More robust logical, kinetic and process models result from simulation of transient data sets because such testing involves wide variations of concentration and accumulation effects that often reveal information on reaction intermediates formed during catalysis. Such data sets are more suitable for testing kinetic and logical models.
Batch-wise coke burning with freshly deposited coke is a transient method and thus rich in kinetic information. However, these batch tests can be time consuming, with one cycle taking an hour or more. Typically, just one cycle is run and then the catalyst is discarded. More testing can be done if a master batch of coked catalyst is made and candidate additives are blended and burned with the master batch. Again, however, just one cycle is run on each sample before the sample is discarded. If fixed fluid bed coke combustion is used on catalyst mixtures which can adsorb and store SOx, oxygen or carbon, a separate step is needed to measure the amount of the adsorbed material. This is not normally practiced, and if it is, the kinetics of the removal of these species are not typically measured.
The information content of steady state pilot plant testing is not particularly high as just one test condition is typically run. This one run can take several hours. The apparatus is expensive to build and operate and it requires large volumes of catalyst and feed. It is not generally thought of as useful for the purpose of screening the performance of experimental catalysts, but is sometimes used for confirmation of performance, the assumption of relevance being implied.
Typical CO promoter testing is done without cracking and coke burning. Instead, feed gases such as CO and O2 diluted in N2 are used, and the steady state catalytic oxidation to CO2 is measured. Sulfur and steam are well known poisons for many catalysts and reactions but these gases are frequently omitted for the sake of convenience. NOx and its precursors are not normally present. Separately, this testing is sometimes conducted at temperatures well below the regenerator temperature, in order to eliminate homogeneous combustion, exotherms, and make the rates easily measured and distinguished. Test results obtained without the important catalyst poisons or at non-representative temperatures run the risk of being misleading or irrelevant.
In addition to the need to assess regenerator environmental additives, the regenerator is thought to cause most of the irreversible catalyst deactivation during FCC. This not only includes zeolite and matrix components of the FCC catalyst, but also deactivation of materials used to trap contaminant metals, of environmental additives used to reduce toxins such as SOx, NOx, CO, and of gasoline sulfur reduction additives. The lifetimes of SOx and CO reduction additives are known to be short in practice, two days or roughly eight hours, respectively, but realistic and convenient deactivation methods proven to be suitable for these additives are lacking. For materials that deactivate quickly, aging methods that operate on the time scale of the regenerator are convenient. For materials that deactivate more slowly, accelerated aging is desirable. Such aging methods would be useful in the development of additives with improved lifetimes, or in the evaluation of additives being considered for purchase and use in a refinery.
Standard methods of aging catalysts include treatment in 100% steam or blends of steam and air, cyclic propylene steaming (CPS) that alternates between oxidation and reduction with propylene, and cracking-regeneration cycles in a fixed fluid bed or circulating pilot plant.
Standard steaming methods for aging catalysts lack oxidation-reduction cycles and do not include reactive gases such as CO2, CO, and SOx. For example, the formation of MgSO4 in a SOx reduction additive could be a key deactivation mechanism in the refinery, but this reaction is excluded in a simple steaming method. CPS steaming includes oxidation-reduction cycles and SOx, but excludes CO2, CO and sulfur from the reducing gas and ensures that propylene and O2 fed during the process do not mix during the cycling. In practice, CO2 may carbonate sodium in the catalyst, reducing the vanadium-sodium synergy which collapses the zeolite in typical procedures. In addition, exclusion of CO—O2 blending prevents the exothermic combustion on CO promoters which could be expected to accelerate CO promoter deactivation. The propylene used in CPS steaming may be too powerful a reducing agent, possibly leading to inappropriate reduction or carburization of metals. Hydrocarbons such as propylene are not majority species in the regenerator environment. Coke is also deposited on the catalyst during the propylene cycle, and in general it is preferred to burn this coke off the catalyst before running the cracking tests. The vanadium present on a catalyst is more active as V(V) to make coke and H2 than as V(III/IV). Therefore one must do the cracking test after ending the CPS deactivation on a reduction cycle and samples subsequently regenerated in situ cannot be used again. To avoid the inconvenience of the initial coke from propylene, one can do a low temperature burnoff to preserve reduced vanadium and low coke and hydrogen. It is not clear that such refinements will improve the predictiveness of the method for other materials however. Methods not requiring arbitrary coke burnoff or oxidation state adjustment would be useful.
Repeated cycles of cracking and regeneration in fixed fluid beds are practiced in the major laboratories, usually with metal-enriched feeds to simulate Ni, V and Fe deposition in the refinery. These cyclic processes are convenient in that a small amount of catalyst is required and the apparatus is only moderately large. The cyclic processes can also be considered relevant in that both oxidation and reduction cycles are present with sulfur and steam. Typically, however, one oxidation-reduction cycle requires about an hour and the deactivation of a sample requires about a day, so a limited number of oxidation-reduction cycles is run. This process does not lend itself towards screening experimental catalysts and is generally used for more comprehensive testing of favorable samples. The same reservations apply to the use of circulating pilot plants for catalyst and metals deactivation, only more so.